1. Technical Field of the Invention
This disclosure relates in general to methods and apparatuses for reducing unwanted electromagnetic emissions.
2. Description of the Related Art
Electromagnetic emissions, in general, may result from any change or variation in current or voltage. Thus, virtually all electronic equipment and components may be a source of electromagnetic emissions. Digital integrated circuits, for example, are one source of electromagnetic emissions due to their frequently changing logic levels. Electromagnetic emissions may also arise from steep increases and decreases in power consumption. These fluctuations in power may occur during certain types of power intensive activity, such as a code upload in a microprocessor. As the code is being loaded, power consumption spikes, then quickly returns to normal after loading is completed. The sharp rise and fall in power consumption can cause electromagnetic emissions to radiate from the microprocessor power lead, leading to a type of emission known as “conducted emissions.”
Conducted emissions, like other electromagnetic emissions, can disrupt or interfere with proper operation of nearby electronic equipment or components. This interference can be particularly problematic for mission-critical applications, such as military applications, where proper operation of electronic equipment is highly crucial to the success or failure of an endeavor in many cases. As a result, various technical standards have been established to limit the level of electromagnetic emissions from electronic equipment and components used in military applications. For example, MIL-STD-461 contains a test procedure, CE101, that requires power leads carrying a voltage of 28 V (volts) DC or less to limit conducted emissions between 30 Hz (Hertz) and 10 KHz (Kilohertz) to less than 100 dB μA (decibel micro-amps) for certain military naval applications.
A number of solutions exist for reducing or eliminating electromagnetic emissions. These solutions generally involve using one or more passive or active filters, or both, to reduce the electromagnetic emissions. Passive filters are composed generally of components that are non-directional and do not provide gain, such as resistors (R), inductors (L), and capacitors (C). Active filters, on the other hand, can contain a combination of passive components as well as components that are directional and provide gain, including amplifiers, diodes, transistors, transformers, and the like. An example of an active filter is disclosed in U.S. Pat. No. 5,319,534 directed to a series-parallel active power line conditioner utilizing reduced-turns-ratio transformer for enhanced peak voltage regulation capability. Similarly, U.S. Pat. No. 5,319,535 discloses an active power line conditioner having capability for rejection of common-mode disturbances.
Currently available filters, however, have proved either impractical or ineffective for certain types of high power applications. In power leads carrying 28 V and 25 A (DC), for example, a passive filter capable of reducing conducted emissions over the frequency range specified in MIL-STD-461 (i.e., between 30 Hz and 10 KHz) would require an inductor of such a large size as to be impractical for most high density circuit board applications. And while active filters are available for the specified frequency range (and beyond), no active filters have been found with a sufficiently high rating for the elevated levels of voltage and current mentioned above.
For some applications, an active filter could be constructed using a transistor connected on the main power line between the power source and a load. However, the normal operation of these filters is such that the voltage and/or current delivered to the load would sag, which may adversely affect proper operation of the load. This type of filter has been known to burn off an equivalent amount of energy as the signal to be filtered. Thus, no active or passive filters have been found that are capable of operating effectively, efficiently, and feasibly at high power levels over the frequency range required by MIL-STD-461.
Accordingly, there exists a need for a filter that can overcome the shortcomings of currently available filters as described above. In particular, there is a need for a filter that can reduce conducted emissions on power leads in the 30 Hz to 10 KHz frequency range, and above, at high power levels.